I activated the plugin successfully. Then I go to the configuration page and i see :

by | Google 404 Plugin - .htaccess tutorial - .htaccess help forum
,NOTE: This is an incredibly powerful plugin as it modifies your server as opposed to modifying wordpress. This can easily take your site down, so check for .htaccess support first. If you experience a bad error, delete the .htaccess files on the server using ftp, ssh, webftp, contact support, etc..
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